Audio coding at low or moderate bitrates is widely used to reduce network load. However, bit rate reduction inevitably leads to quality decrease due to an increased amount of quantization noise. One way to minimize the perceptual impact of quantization noise is to use a post-filter. A post-filter operates at the decoder and affects reconstructed signal parameters, or, directly the signal waveform. The use of a post-filter aims at attenuating spectrum valleys, where quantization noise is most audible, and thereby achieve improved perceptual quality.
Both pitch and formant post-filters are used for quality enhancement in so-called ACELP (Algebraic Code Excited Linear Prediction) speech codecs. These filters operate in the time-domain and are typically based on the speech model used in the ACELP codec [1]. However, this family of post-filters is not well suited for use with transform audio codecs, such as e.g. G.719 [2].
Thus, there is a need for improving the perceptual quality of audio signals which have been subjected to transform audio coding.